Spring是怎么解析Xml配置文件路径中的占位符的

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Spring是怎么解析Xml配置文件路径中的占位符的

2024-07-14 15:41| 来源: 网络整理| 查看: 265

前言

我们在配置Spring Xml配置文件的时候，可以在文件路径字符串中加入 ${} 占位符，Spring会自动帮我们解析占位符，这么神奇的操作Spring是怎么帮我们完成的呢？这篇文章我们就来一步步揭秘。

1.示例

ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext(); applicationContext.setConfigLocation("${java.version}.xml"); applicationContext.refresh(); String[] beanNames = applicationContext.getBeanDefinitionNames(); for (String beanName : beanNames) { System.out.println(beanName); }

这段代码在我工程里是会报错的，如下：

Caused by: java.io.FileNotFoundException: class path resource [1.8.0_144.xml] cannot be opened because it does not exist at org.springframework.core.io.ClassPathResource.getInputStream(ClassPathResource.java:190) at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:336) ... 11 more

可以看到报错里面的文件路径变成了1.8.0_144.xml，也就是说Spring帮我们把${java.version}解析成了实际值。

2.原理 AbstractRefreshableConfigApplicationContext 我们在之前的文章里提到过这个类的resolve方法，我们再来瞧一眼：

/** * Resolve the given path, replacing placeholders with corresponding * environment property values if necessary. Applied to config locations. * @param path the original file path * @return the resolved file path * @see org.springframework.core.env.Environment#resolveRequiredPlaceholders(String) */ protected String resolvePath(String path) { //通过当前环境去解析必要的占位符 return getEnvironment().resolveRequiredPlaceholders(path); }

获取当前环境，这个环境在示例代码中就是 StandardEnvironment ,并且根据当前环境去解析占位符，这个占位符解析不到还会报错。

resolveRequiredPlaceHolders由StandardEnvironment的父类AbstractEnvironment实现。

##AbstractEnvironment

//把propertySources放入 Resolver中 private final ConfigurablePropertyResolver propertyResolver = new PropertySourcesPropertyResolver(this.propertySources); @Override public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException { return this.propertyResolver.resolveRequiredPlaceholders(text); }

这里的propertySources很重要了，从命名也可以看出我们解析占位符的来源就是从这个集合中来的。这个集合是在我们StandardEnvironment实例化的时候去自定义的。

##StandardEnvironment

/** * Create a new {@code Environment} instance, calling back to * {@link #customizePropertySources(MutablePropertySources)} during construction to * allow subclasses to contribute or manipulate(操作) {@link PropertySource} instances as * appropriate. * @see #customizePropertySources(MutablePropertySources) */ //StandardEnvironment 实例化调用 public AbstractEnvironment() { customizePropertySources(this.propertySources); } @Override protected void customizePropertySources(MutablePropertySources propertySources) { //todo Java提供了System类的静态方法getenv()和getProperty()用于返回系统相关的变量与属性， //todo getenv方法返回的变量大多于系统相关， //todo getProperty方法返回的变量大多与java程序有关。 //https://www.cnblogs.com/Baronboy/p/6030443.html propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties())); //SystemEnvironmentPropertySource 是System.getenv() propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment())); }

最重要的肯定是我们的 propertyResolver.resolveRequiredPlaceholders 方法了，propertyResolver.resolveRequiredPlaceholders其实是PropertySourcesPropertyResolver的父类AbstractPropertyResolver来实现。

##AbstractPropertyResolver

//创建一个占位符的helper去解析 @Override public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException { if (this.strictHelper == null) { //不忽略 this.strictHelper = createPlaceholderHelper(false); } return doResolvePlaceholders(text, this.strictHelper); } //私有方法 //是否忽略无法解决的占位符 private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) { //默认使用${ placeholderPrefix return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix, this.valueSeparator, ignoreUnresolvablePlaceholders); } private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) { //PlaceholderResolver function interface //todo important 重要的是这个getPropertyAsRawString return helper.replacePlaceholders(text, this::getPropertyAsRawString); }

这里的 this::getPropertyAsRawString 很重要，利用了java8的函数式接口来实现。它的定义在AbstractPropertyResolver里

/** * Retrieve the specified property as a raw String, * i.e. without resolution of nested placeholders. * @param key the property name to resolve * @return the property value or {@code null} if none found */ @Nullable protected abstract String getPropertyAsRawString(String key);

但是我们在doResolvePlaceholders里指向的this,所以还得看PropertySourcesPropertyResolver类。

##PropertySourcesPropertyResolver

//提供给函数接口 PlaceholderResolver //todo 解析 xml配置文件路径占位符的时候调用的是这个 2020-09-11 @Override @Nullable protected String getPropertyAsRawString(String key) { return getProperty(key, String.class, false); } @Nullable protected T getProperty(String key, Class targetValueType, boolean resolveNestedPlaceholders) { if (this.propertySources != null) { //例如遍历的是MutablePropertySources 的propertySourceList for (PropertySource propertySource : this.propertySources) { if (logger.isTraceEnabled()) { logger.trace("Searching for key '" + key + "' in PropertySource '" + propertySource.getName() + "'"); } Object value = propertySource.getProperty(key); if (value != null) { //todo 解析 profile变量的时候会去解析变量中的占位符 2020-09-11 //TODO 解析xml配置文件路径字符串的时候如果占位符变量的值包含占位符在这里不会去解析通过Helper 去解析 PropertyPlaceholderHelper if (resolveNestedPlaceholders && value instanceof String) { value = resolveNestedPlaceholders((String) value); } logKeyFound(key, propertySource, value); //跳出for 循环 return convertValueIfNecessary(value, targetValueType); } } } if (logger.isTraceEnabled()) { logger.trace("Could not find key '" + key + "' in any property source"); } return null; }

看到没有，我们是遍历this.propertySources集合，然后根据key调用它的getProperty方法获取value。我们从上面的StandardEnvrionment中看到我们定义的是 MapPropertySource 和 SystemEnvironmentPropertySource .

##MapPropertySource

//从source中取得属性 @Override @Nullable public Object getProperty(String name) { return this.source.get(name); }

这里的source就是getSystemProperties()，也就是 AbstractEnvironment中的方法：

@Override @SuppressWarnings({"unchecked", "rawtypes"}) public Map getSystemProperties() { try { //Hashtable return (Map) System.getProperties(); } catch (AccessControlException ex) { return (Map) new ReadOnlySystemAttributesMap() { @Override @Nullable protected String getSystemAttribute(String attributeName) { try { return System.getProperty(attributeName); } catch (AccessControlException ex) { if (logger.isInfoEnabled()) { logger.info("Caught AccessControlException when accessing system property '" + attributeName + "'; its value will be returned [null]. Reason: " + ex.getMessage()); } return null; } } }; } }

我们还忘了很重要的一步，就是PropertyPlaceholderHelper的replacePlaceholders方法。

##PropertyPlaceholderHelper

//protected 范围 protected String parseStringValue( String value, PlaceholderResolver placeholderResolver, Set visitedPlaceholders) { StringBuilder result = new StringBuilder(value); //如果value中没有占位符前缀那直接返回result int startIndex = value.indexOf(this.placeholderPrefix); while (startIndex != -1) { //找到占位符的最后一个索引 int endIndex = findPlaceholderEndIndex(result, startIndex); if (endIndex != -1) { String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex); String originalPlaceholder = placeholder; if (!visitedPlaceholders.add(originalPlaceholder)) { throw new IllegalArgumentException( "Circular placeholder reference '" + originalPlaceholder + "' in property definitions"); } //1. todo 2020-09-01 解析出来占位符，比如java.version //解析内嵌占位符 // Recursive invocation, parsing placeholders contained in the placeholder key. placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders); // Now obtain the value for the fully resolved key... //2.todo 2020-09-01 获取实际值 String propVal = placeholderResolver.resolvePlaceholder(placeholder); if (propVal == null && this.valueSeparator != null) { int separatorIndex = placeholder.indexOf(this.valueSeparator); if (separatorIndex != -1) { String actualPlaceholder = placeholder.substring(0, separatorIndex); String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length()); //这里就是实际获取占位符中值得地方。 propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder); } } if (propVal != null) { //从占位符里获取的值也有可能包含占位符这里可能会报 Circular placeholder reference propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders); //替换占位符为实际值 result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal); if (logger.isTraceEnabled()) { logger.trace("Resolved placeholder '" + placeholder + "'"); } startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length()); } //省略部分代码 } else { startIndex = -1; } } return result.toString(); }

到这里我们就可以看到Spring在处理一个小小的占位符就做了这么多设计。可见这个架构是如此严谨。下篇文章我们就来探讨下Spring是如何加载这个Xml文件的。

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Spring是怎么解析Xml配置文件路径中的占位符的

Spring是怎么解析Xml配置文件路径中的占位符的

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